CS61C:GreatIdeasinComputerArchitecture

Lecture11:RISC-VProcessorDatapath

KrsteAsanović &RandyKatz

http://inst.eecs.berkeley.edu/~cs61c/fa17

Recap:CompleteRV32IISA

2

NotinCS61C

StateRequiredbyRV32IISAEachinstructionreadsandupdatesthisstateduringexecution:• Registers(x0..x31)

− Registerfile(orregfile)Reg holds32registersx32bits/register:Reg[0].. Reg[31]− Firstregisterreadspecifiedbyrs1fieldininstruction− Secondregisterreadspecifiedbyrs2fieldininstruction− Writeregister(destination)specifiedbyrd fieldininstruction− x0 isalways0(writestoReg[0]areignored)

• ProgramCounter(PC)− Holdsaddressofcurrentinstruction

• Memory(MEM)− Holdsbothinstructions&data,inone32-bitbyte-addressedmemoryspace− We’lluseseparatememoriesforinstructions(IMEM)anddata(DMEM)

§ Laterwe’llreplacethesewithinstructionanddatacaches− Instructionsareread(fetched)frominstructionmemory(assumeIMEM read-only)− Load/storeinstructionsaccessdatamemory

10/3/17 3

One-Instruction-Per-CycleRISC-VMachine• Oneverytickoftheclock,thecomputerexecutesoneinstruction

• Currentstateoutputsdrivetheinputstothecombinationallogic,whoseoutputssettlesatthevaluesofthestatebeforethenextclockedge

• Attherisingclockedge,allthestateelementsareupdatedwiththecombinationallogicoutputs,andexecutionmovestothenextclockcycle

CS61c 4

Reg[]

pc

IMEM

DMEM

CombinationalLogic

clock

BasicPhasesofInstructionExecution

IMEM

+4

rs2rs1rd

Reg[]

ALU

DMEM

imm

1.InstructionFetch

2.Decode/RegisterRead

3.Execute 4.Memory 5.RegisterWrite

PC

10/3/17 5

mux

Clocktime

Implementingtheadd instruction

add rd, rs1, rs2• Instructionmakestwochangestomachine’sstate:

− Reg[rd] = Reg[rs1] + Reg[rs2]− PC = PC + 4

CS61c 6

ControlLogic

Datapath foradd

CS61c 7

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0] RegWriteEnable(RegWEn)

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD Reg[rs1]

Reg[rs2]+ alu

TimingDiagramforadd

8

1000 1004PC

1004 1008PC+4

add x1,x2,x3 add x6,x7,x9inst[31:0]

Clock

time

+4

pcpc+4 inst[11:7]

inst[19:15]inst[24:20]

IMEM

inst[31:0]

+RegWEn

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD Reg[rs1]

Reg[rs2]

clock

alu

Reg[2] Reg[7]Reg[rs1]

Reg[2]+Reg[3]alu Reg[7]+Reg[9]

Reg[3] Reg[9]Reg[rs2]

???Reg[1] Reg[2]+Reg[3]

Implementingthesub instruction

sub rd, rs1, rs2• Almostthesameasadd,exceptnowhavetosubtractoperandsinsteadofaddingthem• inst[30] selectsbetweenaddandsubtract

CS61c 9

ControlLogic

Datapath foradd/sub

CS61c 10

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0] RegWEn(1=write,0=nowrite)

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD Reg[rs1]

Reg[rs2]alu

ALU

ALUSel(Add=0/Sub=1)

ImplementingotherR-Formatinstructions

• Allimplementedbydecodingfunct3andfunct7fieldsandselectingappropriateALUfunction

CS61c 11

Implementingtheaddi instruction• RISC-VAssemblyInstruction:

addi x15,x1,-50

1210/3/17

111111001110 00001 000 01111 0010011

OP-Immrd=15ADDimm=-50 rs1=1

ControlLogic

Datapath foradd/sub

CS61c 13

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0] RegWEn(1=write,0=nowrite)

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD Reg[rs1]

Reg[rs2]alu

ALU

ALUSel(Add=0/Sub=1)

ControlLogic

Addingaddi todatapath

CS61c 14

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0]

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Reg[rs1]

Reg[rs2]

aluALU

ALUSel=Add

Imm.Gen

0

1

RegWEn=1

inst[31:20] imm[31:0]

ImmSel=I BSel=1

I-Formatimmediates

CS61c 15

inst[31:0]

------inst[31]-(sign-extension)------- inst[30:20]

imm[31:0]Imm.Gen

inst[31:20] imm[31:0]

ImmSel=I

• High12bitsofinstruction(inst[31:20])copiedtolow12bitsofimmediate(imm[11:0])

• Immediateissign-extendedbycopyingvalueofinst[31]tofilltheupper20bitsoftheimmediatevalue(imm[31:12])

ControlLogic

Addingaddi todatapath

CS61c 16

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0]

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Reg[rs1]

Reg[rs2]

aluALU

ALUSel=Add

Imm.Gen

0

1

RegWEn=1

inst[31:20]imm[31:0]

ImmSel=I BSel=1

AlsoworksforallotherI-formatarithmeticinstruction(slti,sltiu,andi,ori,xori,slli,srli,srai)justbychangingALUSel

TSMCAnnounces3nmCMOSFab

CS61c 17

LatestAppleiPhone8,iPhoneXuseTSMC’s10nmprocesstechnology.

3nmtechnologyshouldallow10xmorestuffonthesamesizedchip(10/3)2

Thenewmanufacturingplantwilloccupynearly200acresandcostaround$15B,openinaround5years(~2022).

Currently,fabs use193nmlighttoexposemasksFor3nm,somelayerswilluseExtremeUltra-Violet(13.5nm)

Break!

10/3/17 18

ImplementingLoadWordinstruction• RISC-VAssemblyInstruction:

lw x14, 8(x2)

1910/3/17

000000001000 00010 010 01110 0000011

LOADrd=14LWimm=+8 rs1=2

ControlLogic

Addingaddi todatapath

CS61c 20

+4

pcpc+4

inst[11:7]

inst[19:15]

inst[24:20]

IMEM

inst[31:0]

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Reg[rs1]

Reg[rs2]

aluALU

ALUSel=Add

Imm.Gen

0

1

RegWEn=1

inst[31:20]imm[31:0]

ImmSel=I BSel=1

Addinglw todatapath

CS61c 21

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr DataR 0

1pc

0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:20]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BSel ALUSel MemRW WBSel

wb

Addinglw todatapath

CS61c 22

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr DataR 0

1pc

0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:20]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel=I RegWEn=1 Bsel=1 ALUSel=Add MemRW=Read WBSel=0

wb

AllRV32LoadInstructions

• Supportingthenarrowerloadsrequiresadditionalcircuitstoextractthecorrectbyte/halfword fromthevalueloadedfrommemory,andsign- orzero-extendtheresultto32bitsbeforewritingbacktoregisterfile.

23

funct3fieldencodessizeandsignedness ofloaddata

ImplementingStoreWordinstruction• RISC-VAssemblyInstruction:

sw x14, 8(x2)

2410/3/17

0000000 01110 00010 010 01000 0100011

STOREoffset[4:0]=8

SWoffset[11:5]=0

rs2=14 rs1=2

combined12-bitoffset=80000000 01000

Addinglw todatapath

CS61c 25

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr DataR 0

1pc

0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:20]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BSel ALUSel MemRW WBSel

wb

Addingsw todatapath

CS61c 26

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR 0

1pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn Bsel ALUSel MemRW WBSel=

wb

Addingsw todatapath

CS61c 27

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR 0

1pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel=S RegWEn=0 Bsel=1 ALUSel=Add MemRW=Write WBSel=*

wb

*=“Don’tCare”

I-Formatimmediates

CS61c 28

inst[31:0]

------inst[31]-(sign-extension)------- inst[30:20]

imm[31:0]Imm.Gen

inst[31:20] imm[31:0]

ImmSel=I

• High12bitsofinstruction(inst[31:20])copiedtolow12bitsofimmediate(imm[11:0])

• Immediateissign-extendedbycopyingvalueofinst[31]tofilltheupper20bitsoftheimmediatevalue(imm[31:12])

I&SImmediateGenerator

CS61c 29

imm[11:5] rs2 rs1 funct3 imm[4:0] S-opcode

imm[11:0] rs1 funct3 rd I-opcode

inst[31](sign-extension) inst[30:25]

imm[31:0]

inst[31:0]

inst[24:20]

SI

inst[31](sign-extension) inst[30:25] inst[11:7]

067111214151920242531

045101131

1 65

5

S

I

• Justneeda5-bitmuxtoselectbetweentwopositionswherelowfivebitsofimmediatecanresideininstruction

• Otherbitsinimmediatearewiredtofixedpositionsininstruction

ImplementingBranches

• B-formatismostlysameasS-Format,withtworegistersources(rs1/rs2)anda12-bitimmediate• Butnowimmediaterepresentsvalues-4096to+4094in2-byteincrements• The12immediatebitsencodeeven 13-bitsignedbyteoffsets(lowestbitofoffsetisalwayszero,sononeedtostoreit)

30

Addingsw todatapath

CS61c 31

IMEMALU

Imm.Gen

+4

DMEM

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR 0

1pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wbpc+4

Reg[rs1]

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn Bsel ALUSel MemRW WBSel=

wb

Addingbranchestodatapath

CS61c 32

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

01

1

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BrUnBrEq BrLT ASelBSel ALUSel MemRW WBSelPCSel

wb

Addingbranchestodatapath

CS61c 33

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

01

1

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel=B RegWEn=0 BrUnBrEq BrLT ASel=1Bsel=1

ALUSel=Add

MemRW=Read WBSel=*PCSel=taken/not-taken

wb

BranchComparator• BrEq =1,ifA=B• BrLT =1,ifA<B• BrUn =1selectsunsignedcomparisonforBrLT,0=signed

• BGEbranch:A>=B,if!(A<B)

CS61c 34

BranchComp.

A

B

BrUn BrEq BrLT

Administrivia (1/2)• Midterm1hasbeengraded!• RegradeRequestswillopentonight

− DuenextTuesday(inoneweek)− Piazzawillexplaintheinstructions

CS61c 35

Administrivia (2/2)• Project1hasbeenreleased

− Part1isduenextMonday− ProjectPartyinCory293onWednesday7-9pm(possiblylaterifneeded)

• Homework2isduethisFridayat11:59pm−Willhelptodothisbeforetheproject!

• NoGuerrillaSessionthisweek—willstartupagainnextTuesday

CS61c 36

Break!

10/3/17 37

MultiplyBranchImmediates byShift?• 12-bitimmediateencodesPC-relativeoffsetof-4096to+4094bytesinmultiplesof2bytes

• Standardapproach:treatimmediateasinrange-2048..+2047,thenshiftleftby1bittomultiplyby2forbranches

CS61c 38

s rs2 rs1 funct3 imm[4:0] B-opcodeimm[10:5]

s imm[10:5] imm[4:0]

s imm[10:5] imm[4:0] 0

sign-extension

sign-extension

S-Immediate

B-Immediate(shiftleftby1)

Eachinstructionimmediatebitcanappearinoneoftwoplacesinoutputimmediatevalue–soneedone2-waymuxperbit

RISC-VBranchImmediates• 12-bitimmediateencodesPC-relativeoffsetof-4096to+4094bytesinmultiplesof2bytes

• RISC-Vapproach:keep11immediatebitsinfixedpositioninoutputvalue,androtateLSBofS-formattobebit12ofB-format

CS61c 39

sign=imm[11] imm[10:5] imm[4:0]

sign=imm[12] imm[10:5] imm[4:1] 0

S-Immediate

B-Immediate(shiftleftby1)

OnlyonebitchangespositionbetweenSandB,soonlyneedasingle-bit2-waymux

imm[11]

RISC-VImmediateEncoding

40

InstructionEncodings,inst[31:0]

32-bitimmediates produced,imm[31:0]

Onlybit7ofinstructionchangesroleinimmediatebetweenSandBUpperbitssign-extendedfrominst[31]always

ImplementingJALR Instruction(I-Format)

• JALRrd,rs,immediate−WritesPC+4toReg[rd](returnaddress)− SetsPC=Reg[rs1]+immediate− Usessameimmediates asarithmeticandloads

§ nomultiplicationby2bytes

41

Addingbranchestodatapath

CS61c 42

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

01

1

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BrUnBrEq BrLT ASelBSel ALUSel MemRW WBSelPCSel

wb

Addingjalr todatapath

CS61c 43

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

0121

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

pc+4alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BrUnBrEq BrLT ASelBSel ALUSel MemRW WBSelPCSel

wb

Addingjalr todatapath

CS61c 44

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

0121

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

pc+4alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel=B RegWEn=1

BrUn=* BrEq=* BrLT=*

Asel=0Bsel=1

ALUSel=Add

MemRW=Read WBSel=2PCSel

wb

Implementingjal Instruction

• JALsavesPC+4inReg[rd](thereturnaddress)• SetPC=PC+offset(PC-relativejump)• Targetsomewherewithin±219 locations,2bytesapart

− ±218 32-bitinstructions• Immediateencodingoptimizedsimilarlytobranchinstructiontoreducehardwarecost

45

Addingjal todatapath

CS61c 46

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

0121

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

pc+4alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BrUnBrEq BrLT ASelBSel ALUSel MemRW WBSelPCSel

wb

Addingjal todatapath

CS61c 47

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

0121

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

pc+4alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel=J RegWEn=1

BrUn=* BrEq=* BrLT=*

Asel=1Bsel=1

ALUSel=Add

MemRW=Read WBSel=2PCSel

wb

Single-CycleRISC-VRV32IDatapath

CS61c 48

IMEMALU

Imm.Gen

+4

DMEMBranchComp.

Reg[]

AddrAAddrB

DataAAddrD

DataB

DataD

Addr

DataWDataR

1

0

0121

0 pc0

1

inst[11:7]

inst[19:15]

inst[24:20]

inst[31:7]

pc+4alu

mem

wb

alu

pc+4

Reg[rs1]

pc

imm[31:0]

Reg[rs2]

inst[31:0] ImmSel RegWEn BrUnBrEq BrLT ASelBSel ALUSel MemRW WBSelPCSel

wb

AndinConclusion,…• Universaldatapath

− CapableofexecutingallRISC-Vinstructionsinonecycleeach− Notallunits(hardware)usedbyallinstructions

• 5Phasesofexecution− IF,ID,EX,MEM,WB− Notallinstructionsareactiveinallphases

• Controllerspecifieshowtoexecuteinstructions−whatnewinstructionscanbeaddedwithjustmostcontrol?

CS61c 49